1. Field of the Invention
The present invention relates to a developing roller for use in an electrophotographic image forming apparatus. Examples of the image forming apparatus include laser printers, electrostatic copying machines, plain paper facsimile machines and printer-copier-facsimile multifunction machines.
2. Description of Related Art
In the various electrophotographic image forming apparatuses described above, a developing roller and a charging blade (layer regulating blade) kept in press contact with an outer peripheral surface of the developing roller are used for developing an electrostatic latent image formed by exposing a surface of an electrically charged photoreceptor drum into a toner image.
That is, when the developing roller is rotated in press contact with the charging blade, toner is electrically charged. Then, the electrically charged toner adheres to the outer peripheral surface of the developing roller, and the amount of the toner adhering to the outer peripheral surface is regulated by the charging blade. Thus, a toner layer having a generally even thickness is formed on the generally entire outer peripheral surface of the developing roller.
In this state, the developing roller is further rotated to transport the toner layer to the vicinity of the surface of the photoreceptor drum. Then, the toner of the toner layer is selectively transferred onto the surface of the photoreceptor drum according to the electrostatic latent image formed on the surface. Thus, the electrostatic latent image is developed into the toner image.
The charging blade is liable to generate frictional heat due to friction between the rotating developing roller and the charging blade. Therefore, the toner is liable to fuse and adhere to the charging blade due to the frictional heat, so that a formed image is liable to suffer from an imaging failure which is referred to as white streaking.
More specifically, if the toner fuses and adheres to a portion of an edge of the charging blade kept in press contact with the developing roller, the developing roller is liable to have a linear surface portion not formed with the toner layer at a position corresponding to the toner adhering edge portion of the charging blade during the rotation of the developing roller. The surface portion of the developing roller not formed with the toner layer results in a white streak on the formed image.
Various measures have been proposed for prevention of the white streaking.
Patent Literature 1 (JP-2001-255737A), for example, proposes that at least a part of the charging blade kept in press contact with the surface of the developing roller is formed of a lower rebound material to reduce the friction for the prevention of the fusion/adhesion of the toner to the charging blade and the prevention of the associated white streaking.
Patent Literature 2 (JP-2008-145885A) proposes that the range of the rubber hardness of the developing roller, a linear abutment load to be applied to the developing roller by the charging blade and other factors are each properly limited for the prevention of the fusion/adhesion of the toner to the charging blade and the prevention of the associated white streaking.
Similarly, Patent Literature 3 (JP-2000-338776A) contemplates that the range of the rubber hardness of the developing roller, a linear abutment load to be applied to the developing roller by the charging blade and other factors are each properly limited to stabilize a developing ability and a cleaning ability in a cleaning-free image forming process.
Patent Literature 4 (JP-2007-164082A) proposes that three types of external additives having different properties are used for coating toner particles for the prevention of the fusion/adhesion of the toner to the charging blade and the like.
Patent Literature 5 (JP-2009-150949A) proposes that a toner capturing/recovering member for capturing smaller-diameter toner particles more liable to fuse and adhere to the charging blade is provided in a developing device for the prevention of the fusion/adhesion of the toner to the charging blade and the like.